JP Morgan boss Jamie Dimon had his bonus cut in half but has escaped harsh criticism from the bank's internal reports into its multi-billion dollar London trading fiasco.

As the bank reported record profits, JP Morgan released two reports into last May's massive trading debacle. Dimon "could have better tested his reliance on what he was told", the reports conclude. Dimon's total compensation has been cut in the wake of the London disaster to $11.5m from $23m for the year before.

In a conference call with analysts Dimon said the bank was "getting near the end" of unwinding the losses, which now exceed $6.2bn. "Obviously when you have a problem like the whale, you have mistakes which you should fix, which we have done. Some of these mistakes obviously scared us," said Dimon.

The internal reviews concluded that Dimon bore "ultimate responsibility" for the losses but saved their most stinging criticism for Ina Drew, the former chief investment officer, and other executives since ousted from the bank.Drew's "judgment, execution and escalation of issues in the first quarter of 2012 were poor, in at least six critical areas". Drew was once one of Dimon's closest allies, and he initially defended her performance.

Among its criticisms, the report found that Drew failed to properly oversee the portfolio that led to the losses; failed to ensure its risk measurements worked properly; and failed to "appreciate the magnitude and significance of the changes" that unit was dealing with.

The bank found that the London traders' strategies "were poorly conceived and not fully understood." The reports also singled out Barry Zubrow, the bank's former chief risk officer, who was also singled out for "significant responsibility for failures." Douglas Braunstein, the former chief financial officer who initially said the bank was "very comfortable" with its position, was cited "for weaknesses in financial controls" and "failure to have asked more questions".

Bruno Iksil, the trader who became known as the London Whale or Vodermort for his huge and ultimately disastrous bets on the derivatives market, is not mentioned by name in either report.

The losses were initially dismissed by Dimon as a "tempest in a teapot", but they sparked government inquiries on both sides of the Atlantic, led to calls for tighter regulation of banks by President Obama.

"Mr Dimon bears some responsibility for that. Importantly, once Mr Dimon became aware of the seriousness of the issues presented by the CIO, he responded forcefully by directing a thorough review and an internal program of remediation. Mr Dimon reports to the board, and the board will weigh the extent of Mr Dimon's responsibility", the report states.

The report's release came as JP Morgan released fourth-quarter earnings that demonstrate once more how little damage America's biggest bank's balance sheet suffered from the massive losses.

Net income jumped 53% to $5.7bn as the bank made more mortgages and saw a decline in costs for bad loans. Revenues, too, were strong, rising 10% to $23.7bn. Dimon said performance had been strong "across virtually all our businesses".